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Planetary pile-up possible in next five billion years

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 16 June, 2009

According to new simulations, the evolution of the Solar System’s inner planets' orbits could lead to a planetary pile-up within the lifetime of our Sun.

It’s certainly happened before – the formation of our own Moon spawned from the collision of a Mars-sized body with the young Earth, and the Solar System’s inhabitants are constantly at risk of a bump by a passing asteroid that strays too close. But what of planets careering around the Solar System like an out-of-control pinball game?

There is a one percent chance that collisions between the inner planets will occur within the lifetime of our Sun. Image: NASA/JPL-Caltech/T. Pyle (SSC/Caltech).

Jacques Lasker and Mickael Gastineau of the Paris Observatory have been working on the problem of predicting how planetary orbits will evolve in the future. Using the most detailed information available about the position and orbital velocity of the Solar System’s major bodies, including our own Moon, the duo conducted simulations of how the orbits could change over the predicted life time of our Sun, five billion years.

Some 2,500 simulations later, Lasker and Gastineau show that the steady dance of the planets around the Sun can be disturbed by the unrelenting gravitational attractions between the planets, such that there is a one percent chance that utter chaos will ensue before the Sun reaches the end of its lifetime. The culprit, surprisingly, is small and unsuspecting Mercury.

In spite of its diminutive size, should Mercury become trapped in a particular resonance with Jupiter, its journey around the Sun would become so eccentric that it could either fall into the Sun itself or meet Venus in a head-on collision. In one simulation, a Mercury-Venus collision happens as early as 1.76 billion years from now.

Perhaps more surprising, a subsequent decrease in Mercury’s eccentricity from an initially high-eccentricity state results in a transfer of angular momentum from the giant planets that destabilises the orbits of all the terrestrial planets, 3.34 billion years from now, resulting in numerous possible collision scenarios between Mercury, Mars or Venus with the Earth. One solution saw Mars approach the Earth with just 800 kilometres to spare. Such a close encounter would cause extreme tidal stress on the Red Planet, ripping it apart and sending a curtain of rocky debris crashing down onto our planet, devastating any life still existing on the Earth.

A collision between Venus and Earth or Mercury and Earth could also occur, and in one extreme scenario Mars was ejected from the Solar System altogether.

Despite the interplanetary melee that a number of the simulations predicted, the future isn’t as bad as it may sound, since there is a 99 percent chance that the orbits of the four inner rocky planets of the Solar System will continue like clockwork for the next five billion years. Of course, around this time, our Sun will evolve into a red giant star to gobble up the rocky planets anyway.

Further details of the simulations and evolution of the planetary orbits can be found in Lasker and Gastineau’s paper Existence of collisional trajectories of Mercury, Mars and Venus with the Earth, published in the 11 June edition of the journal Nature.

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